Owing to ongoing technological developments in the coating process, there is a continuing demand for new developments in the field of pressure sensitive adhesives (PSAs). In industry, hotmelt processes with solvent-free coating technology for preparing PSAs are of growing significance, since the environmental strictures are becoming ever greater and the prices of solvents are also rising. Hotmelt processes are already state of the art for SIS adhesives. In contrast, acrylate PSAs are still processed largely from solution. In this respect, an excessive average molecular weight continues to present problems, since, although it is essential for high shear strength, it causes a sharp rise in the flow viscosity, and so acrylate hotmelts with an average molecular weight of >1 000 000 g/mol are difficult to process from the melt.
On the other hand, low molecular weight acrylate hotmelts have already been successfully implemented as hotmelt PSAs (BASF AG, e.g. UV 203 AC Resins). Here, benzophenone derivatives or acetophenone derivatives are incorporated as an acrylated photoinitiator into the acrylate polymer chain and are then crosslinked with UV radiation [U.S. Pat. No. 5,073,611]. Nevertheless, the achievable shear strength for systems of this kind is still not satisfactory, although, as a result of the low average molecular weight
(≈250 000 g/mol), the flow viscosity is relatively low.
The preparation of relatively high molecular weight acrylate PSAs (average molecular weight between 250 000 g/mol and 1 000 000 g/mol) necessitates specific polymerization processes. Polymerization cannot be carried out without solvent, since at a certain point in time the flow viscosity becomes too high and the conversion of the reaction is very low. The residual monomers remaining would disrupt the hotmelt operation. Consequently, acrylate monomers are polymerized conventionally in solution and then concentrated in a concentrating extruder [EP 0621 326 B1].
Nevertheless, the concentration of this acrylate PSA causes problems, since environmental considerations frequently dictate the use of solvent mixtures, such as special-boiling-point spirit and acetone (state of the art). Toluene is suspected of being carcinogenic, and is therefore no longer used. In a concentrating operation, a solvent mixture means that there is no continuous boiling point, and so it is very difficult to remove the solvent down to a fraction of below 0.5% (percent by weight based on the polymer). Attempts are therefore made to polymerize acrylates in only one solvent and with one regulator. The regulator fulfils the functions of avoiding gelling, lowering the average molecular weight, absorbing the heat given off in the initiation phase, reducing the molecular weight distribution, and yet ensuring a high conversion.
The regulators used are generally thiols, alcohols or halides, such as carbon tetrabromide, for example [cf., for example, H.-G. Elias, “Makromoleküle”, Hüthig & Wepf Verlag Basel 5th edition, 1990]. The use of halide regulators, as described in U.S. Pat. No. 7,034,085 B2, for example, is decreasing persistently on environmental grounds. Thiols and alcohols are suitable as regulators and, depending on concentration, greatly reduce the average molecular weight of the polymer. Often, however, they have the disadvantage of being volatile and are therefore located in the distillate following the removal of the solvent mixture. This results in the disadvantages that reusing the solvent mixture removed by distillation for further polymerizations leads to an accumulation of the compound with chain-transfer regulator activity, and hence that a reproducible molar mass distribution is not ensured as the recycling rate goes up.
US 20070299226 A1 describes various chain transfer regulators based on a thiol functionality. They do remain partly in the polymer, but have the disadvantage that they are partly toxic and, on account of their strong and unpleasant odour, are not suitable when the polymer is used for a product, such as a pressure-sensitive adhesive tape, for example, with which points of contact in everyday use are frequent. Moreover, the same specification describes polyfunctional thiols for use as regulators, but such thiols may even lead to crosslinking of the polymer and hence to high melt viscosities, thereby making it no longer possible to carry out processing from the melt. The mercapto-functionalized photoinitiators that are described in EP 1 311 554 B1 likewise result in crosslinking or make it necessary to ensure that the polymer prior to processing is not exposed to light or any other electromagnetic radiation, since otherwise there may be crosslinking.
It is an object of the invention to optimize the preparation of polyacrylates in respect of recovery of the solvent.
It should advantageously be possible to obtain polyacrylate compositions having sufficiently high average molecular weights that they can be used, for example, as pressure sensitive adhesives. The solvent mixture or solvent used in preparing the polyacrylates ought preferably to be able to be reused without purification steps following distillative removal; and the capacity for the polyacrylate to be processed in a hotmelt operation ought to be ensured.